A speech encoding technology that compresses a speech signal or audio signal at a low bit rate is important to effectively use transmission path capacity in a communication system. In recent years, as principal application of the speech encoding technology, communication systems typified by a VoIP (Voice over IP) network and mobile telephone network draw attention. VoIP is a speech communication technology that uses a packet communication network using IP (Internet Protocol) stores an encoded code of a speech signal in a packet, and exchanges packets with a communicating party.
In the speech communication system, in order to establish speech communication with the communicating party, the communication terminal apparatus that the user has have to accurately interpret and implement decoding processing of the encoded code generated by the communication terminal apparatus that the communicating party has. Therefore, after deciding the specification of a codec for the speech communication system once, it is not easy to change this specification. This is because, if the specification of the codec is tried to be changed, it is necessary to change the functions of both encoding apparatus and decoding apparatus. When it is considered that some kind of a new extension function is provided to the encoding apparatus, and information about the extension function is transmitted together, it is necessary to revise the specification of the codec of the speech communication system, and therefore a cost increases substantially.
In patent document 1 or non-patent document 1, speech encoding methods of embedding additional information in an encoded code using the steganographic technology are disclosed. For example, even if the least significant bit of the encoded code is changed to some extent, a person cannot auditorily perceive the difference. In order to add new information at a transmission apparatus, bits indicating additional information are embedded in the least significant bit of speech data that does not cause auditory problems, and this data is transmitted. According to this technology, even if the encoding apparatus is provided with some kind of an extension function, and information about this extension function is embedded in the original encoded code as an extension code and transmitted, there is no case where the decoding apparatus cannot perform decoding. Namely, it is possible to interpret this encoded code and generate a decoding signal at the decoding apparatus that is not compatible with the extension function as well as at the decoding apparatus compatible with the extension function.
For example, in the above-described patent document 1, as information about the above-described extension function, information for applying a compensation technology for suppressing deterioration in speech quality due to a packet loss etc. is embedded, and further, in the above-described non-patent document 1, information for extending a narrow band signal to a wide band signal is embedded.
Patent Document 1: Japanese Patent Application Laid-open No. 2003-316670.
Non-patent document 1: Aoki et. al., “A band widening technique for VoIP speech using steganography”, IEICE Technical Report, SP2003-72, pp. 49-52.